Have you ever considered using only potassium iodide?
Iodide can be taken in huge doses compared with iodine, for ex 5gms per day.
Also, it is important to take 1,500 vitamin C two times per day to support the sodium-iodide symporter (NIS).
Some people have problems with Iodide getting where needs due to a malfunction in the NIS.
I don't have the info on which iodine/iodide is prefered by the kidneys, but this study shows it uses iodide. http://www.nature.com/ki/journal/v59/n3/full/4492116a.html